The present invention relates generally to imaging sensors, and more particularly, to scene based nonuniformity correction methods for use with such imaging sensors.
Nonuniformities appear at an output display of an imaging sensor as fixed pattern noise. The Nonuniformities are described as noise because they result in undesirable information. The nonuniformities are described as a fixed pattern because their characteristics do not change (or change relatively slowly) with time. These nonuniformities may also be thought of as detector gain and offset errors. In the method of the present invention, all errors are treated as offset errors. Thus, the present invention accurately measures the detector offsets using actual scene information.
Once the offset errors have been measured there are several ways in which the corrections may be applied. They may be used as the only source of error correction. They may also be used as fine offset correction terms, in conjunction with coarse offset terms and gain correction terms. These other terms may be calculated using a number of different methods. These methods include coarse offset terms calculated using a thermal reference source; coarse offset and gain terms calculated as part of system initialization; and fine gain terms calculated using thermal reference sources or scene-based methods.
In one current method employed by the assignee of the present invention, one or more thermal reference sources are used to measure nonuniformities for a scanning infrared sensor and provide data for the calculation of correction coefficients that are employed to correct for the nonuniformities. There are several disadvantages related to the use of a thermal reference source-based correction system. First, there is added mechanical complexity which leads to increased system cost. Secondly, system performance may suffer.
System performance suffers for two reasons. In many cases, a separate optical path is utilized for each thermal reference source. Thus, the correction coefficients calculated using the thermal reference source optical path may not be the proper ones for the optical path of the scanning infrared sensor. This leads to imperfect correction. In less sophisticated systems, the temperature of the thermal reference source cannot be controlled. In this case, the thermal reference source may not be at the same temperature as tile scene that is viewed. The correction coefficients thus correspond to the wrong part of the detector response curve. This also leads to imperfect correction. The present method avoids these problems by using scene temperature information. Furthermore, the present invention does not degrade the scene in any manner.